The present invention relates to processes for controlling odor in paper sludges and other papermaking waste, and more particularly relates to controlling and/or suppressing odor in paper sludges using cyclodextrins, and also the resulting deodorized sludges and composite products that incorporate the deodorized sludges.
The conventional manufacture of paper, cardboard, and related paper products typically results in large quantities of papermaking sludge as a by product. Large paper mills may produce many tons of sludge per day. The disposal of the sludge has been a significant concern for the papermaking industry.
The sludge by product arises during papermaking in the following general manner. Paper is conventionally made by draining a low consistency slurry of cellulose fiber pulp, fillers (e.g., clay), and additives (e.g., calcium carbonate or other materials as retention aids, latex dyes, pigments, defoamers, mixing catalysts such as casein or soy protein, etc.) through a paper machine “wire” (essentially an endless mesh or sieve). A certain amount of solid material passes through the wire with the suspension water, and thus it is not retained in the wet paper web formed on the wire. The drained suspension water and suspended solid material is commonly referred to as “white water,” and is reused as far as practicable. However, complete recovery and reuse of this water and papermaking raw materials is not feasible, and a certain proportion is discharged from the paper machine as a waste water stream. This waste water stream from the paper machine typically is treated before it can be discharged from the paper mill. This treatment normally involves passage of the waste water stream through a clarifier or into a settling tank in which heavier components settle to the bottom of the tank and are drawn off as “sludge”. The settled sludge is commonly referred to as the primary sludge. Papermaking sludges are generally divided into two types: primary and secondary. The primary sludge is partly dewatered (e.g., mechanical pressing via a belt filter press, vacuum filter, or screw press), which partly reduces the water content, e.g., to approximately 50% to 80% water content, providing a quantity of sludge for final disposal. Dewatered primary sludges may be dried in a rotary drum dryer, and the like. Secondary sludge is comprised of those particles which are too small to settle in any reasonable length of time from the waste water stream. Secondary sludge consists of colloidal and colloidal-sized particles. Secondary sludge has the consistency of gelatin and mechanical dewatering means are largely ineffective. The secondary sludge may be precipitated in a clarifier by use of a flocculent to precipitate the secondary sludge particles by the process of coagulation, that is, by binding together the minute particles to form flocs which are of sufficient size to precipitate in a clarifier. The secondary sludge may be treated, e.g., with flocculants to promote settling and recovery of this sludge content.
The resulting papermaking sludges often contain large amounts of water, wood fibers, wood particles (“fines”), fillers, additives, together with miscellaneous debris such as grit, sand, plastic particles, dirt, and, particularly if waste paper has been used by the mill as a raw material, ink particles, accumulations of adhesives and other particles used in the paper making process. There is no precise composition for this sludge because there are substantial variations in the wood and other feedstocks used; in the processing materials which must be used to make different types of paper products; and even considerable variation in the processes used by different paper makers in making similar products. This sludge is known in the industry by a variety of names, including: “primary waste treatment sludge”, “paper mill sludge”, “process residual”, “waste treatment sludge”, “waste treatment plant (‘WTP’) sludge”, to name a few. For simplicity, the term “papermaking sludge” is used herein to denote this product, or merely as “sludge”.
As indicated, the disposal of this papermaking sludge is a continuing problem for the paper manufacturers. Paper sludge has traditionally been disposed of, e.g., by landfilling, composting, utilization by the cement industry, and by incineration. However, suitable landfills may be difficult to locate and access for disposal of these sludges. Incineration is only a partial solution since ash deposits still must be handled and the incineration entails added costs of its own. The other conventional disposal approaches also have limitations. Attempts have been made to find economic uses for paper mill sludges which avoid the need for landfill or other disposal, and/or to recover reusable raw materials from the sludges. Some sludges can be burned for steam and/or power generation, but the practicality of this depends on the nature, amount and variability of the sludge produced. For example, sludges with high filler content, as might be produced in a paper mill specializing in fine paper production, may not be adequately combustible, and/or the amount of sludge available may not justify investment in a suitable combustion plant or adaptation of existing plants.
Another problem associated with papermaking sludge is odor. Papermaking sludge is a significant odor source. Paper sludge is typically deficient in oxygen, resulting in an environment ideal for the growth of anaerobic bacteria. These bacteria produce hydrogen sulfide, volatile fatty acid gases, mercaptans, and other undesirable gases. Even if substantially dried, ambient moisture conditions still can promote and sustain discernible odor development within sludge mass. Odor control of papermaking sludges may be attempted with harsh non-biodegradable materials such as chlorine or chlorinated compounds, enzymes, hydrogen peroxide, or other peroxygen containing materials where the application cost may be prohibitive and/or the result is often only temporary.
Cyclodextrins have been proposed for use in treating odors in household/textile applications for spray on fabric fresheners and as combined with textile fibers for clothing, but these environments have not involved dense anaerobic bioactive masses.